A. Prior Art Processes
In the 1960's, Thomas Nicklin and others devised a method for continuous removal of hydrogen sulfide from sour gas streams using an aqueous alkaline absorption solution containing sodium salts of anthraquinone disulfonic acids resulting in the precipitation of elemental sulfur. During this work, it was found that addition of sodium vanadates to the anthraquinone disulfonic acid constituents improved the spread of conversion of hydrogen sulfide to sulfur and increased the hydrosulfide loading of the washing agent. A comprehensive disclosure of the method (referred to as the Stretford Process) is contained in an article by Nicklin and E. Brunner published during the 98th General Meeting of the Institute of Gas Engineers meeting at London, England, May 16-19, 1961.
A number of patents have issued since that time including U.S. Pat. No. 3,035,889 of May 22, 1962 which teaches that when a gas containing hydrogen sulfide is washed with an aqueous alkaline solution of one or more anthraquinone disulfonic acids, the hydrogen sulfide is oxidized liberating free sulfur as the anthraquinone disulfonic acid constituents are reduced. Recovery of the elemental sulfur is disclosed as being feasible in conjunction with reoxidization of the anthraquinone disulfonic acids by passage of an oxygen containing gas therethrough.
In U.S. Pat. No. 3,097,926 of Feb. 16, 1963 Niklin, et al. suggest that gases containing hydrogen sulfide may be treated to convert the H.sub.2 S to elemental sulfur by bringing this sour gas into contact with a liquid alkaline absorption solution containing a metal vanadate such as sodium metavanadate, sodium orthovanadate or sodium ammonium vanadate. In the course of this reaction the vanadate is reduced and the [HS.sup.- ] is oxidized to free sulfur. In the '926 Patent, it is also suggested that a chelating or sequestering agent such as sodium potassium tartrate or other soluble tartrate or tartaric acid or ethylene diamine tetracetic acid (EDTA) may be included with the vanadate to enhance the solubility of the vanadate in the presence of [HS.sup.- ]. It is further indicated that iron salts such as ferrous sulfate or ferric chloride or salts of copper, manganese, chromium, nickel and cobalt may be incorporated in the solution in conjunction with a chelating agent such as EDTA or one of its derivatives, a tartrate or tartaric acid to enhance oxidation of the reduced vanadate during regeneration of the solution.
In U.S. Pat. No. 4,049,776 of Sept. 20, 1977, Nicklin, et al. suggest preparing a ferric ion sulfonated hydroxyl anthraquinone disulfonic acid complex at a pH below 5 and then adding this mixture to the main body of a vanadium-anthraquinone disulfonic acid system thereby precluding precipitation of iron hydroxide in the absorption medium.
Humphreys & Glassgow Limited received United Kindom Pat. Nos. 999,799 and 999,800 of July 28, 1965 describing a process for removal of hydrogen sulfide from sour gas in which the gas is washed with a solution of chelated iron of such nature that it is able to exist dissolved in the solution in both the ferric and ferrous states. Thus, the ferric ion upon reaction with H.sub.2 S is reduced to the ferrous state as the hydrosulfide is oxidized to elemental sulfur. The ferrous iron may then be oxidized back to the ferric state using an agent such as air. Exemplary chelating agents for the iron include a number of amino acids preferably of the ethylene or trimethylene group.